1. Field of the Invention
The present invention generally relates to wireless communication systems, and more particularly to a wireless communication system using transmit and receive antenna arrays to increase information transfer rate in the system.
2. Description of Related Art
An ordinary point-to point wireless link typically includes a single antenna at the transmitter and a single antenna at the receiver. A typical point-to-point wireless link may include a base station and a mobile station with transmissions from the base to the mobile occurring on what is characterized as a downlink and transmissions from the mobile to the base occurring on an uplink. One or both antennas may be highly directional in order to maximize received radio power. The maximum possible rate at which data signaling over such a radio or “air interface” link can occur is well understood, and depends on factors such as transmission frequency, the available bandwidth and the signal-to noise ratio as experienced at the receiver. Thus an idealized link in a wireless radio communication system might include a transmitter and a receiver each with highly directional antennas oriented toward each other and transmitting at a high frequency for maximum information transfer. An idealized link however is not what is typically found in most radio communication environments.
Most modern mobile radio communication is based on the cellular concept, in which a mobile terminal communicates with the nearest base station enabling transmission to occur both on the uplink and the downlink with the lowest possible power. Transmitting with low power is desirable for reasons including conservation of battery in mobile handsets and reduction in interference between channels. In addition, transmit power is often set in various specifications which govern the specific details of cellular radio communications.
At present there are primarily two broad system concepts employed in the majority of cellular radio communications systems: time division multiple access (TDMA) and code division multiple access (CDMA). In TDMA-based systems such as the standards based Global System for Mobile communications (GSM) and the North American TDMA (IS-136), different groups of frequency bands are typically allocated to different cells, and time slots are assigned to distinguish users within a cell. Since only a limited number of frequencies are available, the same communications frequencies are “re-used” in cells located at a predetermined geographical distance from any other cell where the frequency or frequencies are also being used. The geographical distance is made large enough to avoid frequency interference between cells. In stark contrast, CDMA systems use the same frequency band all cells. To avoid interference, spread spectrum modulation is used on transmitted sequences such that transmissions from different users are uniquely “coded”.
In existing systems however, whether TDMA or CDMA, throughput is limited since typically, a single fixed resource is divided according to the number of users. In the case of TDMA, throughput may be limited by division of the time domain according to the number of users and the number of frequencies, and with CDMA throughput may be limited by the number of unique codes that may be assigned according to a number of users. Additionally there is a need for increasing the throughput on limited resources beyond that provided by conventional multiple access methods.